1. Field of the Invention
The present invention relates generally to a substrate for a semiconductor device and a semiconductor device, and more particularly, to a substrate for a semiconductor device and a semiconductor device substantially reduced to a chip size suitable for high density packaging.
2. Description of the Background Art
Chip-size Package (CSP) semiconductor devices such as QFP (Quad Flat Package) type or BGA (Ball Grid Array) type devices have been in wide use to cope with the recent trend toward lighter and more compact apparatus in the market of electrical appliances and adapt to automated assembly process. Increased speed and number of functions in signal processing by semiconductor elements incorporated in these semiconductor devices require a larger number of external connection terminals.
In such a case, a BGA type device having external connection terminals arranged two-dimensionally at the bottom of the semiconductor device is employed. Some devices are formed to be as small as possible so that they can be incorporated into compact mobile equipment, in other words they are formed to have a size close to a chip size. There is a conventional resin encapsulation type semiconductor device as disclosed in Japanese Patent Laying-Open No. 9-121002. In such devices, a semiconductor chip is mounted on an interconnection substrate, electrical conduction with the interconnection substrate is secured by wire-bonding, and then resin encapsulation is achieved to protect the semiconductor chip, wiring and the like. External connection terminals are formed by reflow or the like.
A semiconductor device having this structure is mounted by reflow onto a wiling board such as a printed circuit board for an actual electronic device. When these external connection terminals are formed or when the semiconductor device is mounted onto the wiring board by reflow, moisture in the semiconductor device could evaporate and expand. In order to solve the problem, as shown in FIG. 7, a plan view of a conventional semiconductor device from the surface to form an external connection terminal, a second through hole 19 to discharge moisture remaining between the semiconductor chip of the semiconductor device and an insulating substrate 15 is provided in insulating substrate 15 in a region between a plurality of first through holes 18 where external connection terminals are formed. Note that a land 17 for connecting an external connection terminal is exposed from first through hole 18.
After the mounting by reflow, stress is generated at the connection portion between the semiconductor device and the wiring board because of their different line expansion coefficients or the like in a heating cycle or the like. The above-described semiconductor device has one side of semiconductor chip 1 encapsulated with mold resin 2, and therefore a bowing part forms in the semiconductor device as shown in FIG. 6, if the temperature changes because of a phenomenon characteristic to a bimetal-like structure between semiconductor chip 1 and mold resin 2. FIG. 6 is a view showing stress caused between mounting board 10 and the semiconductor device. The semiconductor device is mounted to mounting board 10 by an external connection terminal 4.
The difference in the above line expansion coefficient or the bowing of the semiconductor device could cause cracks at the connection portion between the semiconductor device and the wiring board, leading to breaking in some cases. At this time, if there are small holes (second through holes) to prevent the trouble associated with reflow as described above, the circular shape of the small holes could not provide enough stress alleviation effect, and therefore a trouble could be caused in a heating cycle following the mounting. Such conventional small holes are formed by drilling and therefore have a circular shape.